A superconductor is a matter having a characteristic that under a certain condition, electric resistance completely disappears and a characteristic of perfect diamagnetism repelling a magnetic field. Such superconductor has been applied to many fields, such as medical service, transport, electronics, electric power, high-energy physics, machine, etc. In particular, a superconductive cable, to which a superconductor is adapted, has excellent transmission efficiency and capacity 5˜10 times those of existing power cable so that it is substituting the existing power cable.
A currently available superconductive cable is generally composed of a combination of a core having a superconductor and a cryostat keeping the core in a cryogenic state. An electric current flows through the superconductor of the core. To secure excellent properties in such superconductive cable, an alternating current loss, such as eddy current loss, should be minimized and the cryogenic state in the superconductive cable should be kept stable.
Japanese Patent Unexamined Publication No. 2005-100777 has proposed a superconductive cable in which a plurality of superconductive wire rods is arranged on an outer circumferential face of a former constituting a core, whereby upon an accident such as a short-circuit, a superconductive layer is prohibited from increasing in temperature, and upon normal current passage, an alternating current loss is reduced. However, in the superconductive cable disclosed in the document, a magnetic field generated by the current flowing through the superconductive layer is applied to the former so that an eddy current is generated in the former, causing AC loss due to the eddy current.
Meanwhile, a superconductive cable is designed such that a transmission current flows through only the superconductive layer because of high resistance of the former at a normal state. In case of low resistance of the former, impedance of the superconductive layer should be kept smaller than the former, so that in order to lower the impedance of the superconductive layer, a strand pitch of the superconductive wire rod should be increased. However, if the strand pitch of the superconductive wire rod is increased, a bending characteristic of the superconductive cable is problematically degraded.